In the following background discussion, as well as in the disclosure of the present invention, the following abbreviations will be frequently used:
BLbrush-lessDCdirect currentESDemergency shutdownLVDTlinear variable differential transformerPMpermanent magnetPSDproduction shutdownSILinstrumented safety levelSMAshape memory alloyVSDvariable speed driveXMT, Xmas treeChristmas tree
The prior art gate valve actuators for hydrocarbon production comprises both hydraulic and electrical control. In the context of the present invention the electrical actuators are the most relevant among the prior art devices.
The concept of using a rotary electrical motor as prime driver and converting this rotary motion to linear motion for operation of a slab gate valve is common. In contemporary designs there is a tendency to select BL DC PM motors as motor technology for high torque performance, and a planetary roller screw for mechanical rotary-to-linear conversion, although other motor types and other rotary-to-linear conversion systems are also commonplace.
A critical feature of slab gate valves and actuators used for control of flow of hydrocarbons through a sub sea Christmas tree is the mechanism provided for emergency operation to the safe position. Upon a failure of the power supply the valve must still revert from the production position (less safe position) to the safe position (no production position).
U.S. Pat. No. 7,172,169 (Biester) and U.S. Pat. No. 6,572,076 (Appleford) are considered representative for contemporary designs and a good example of current efforts in this area both in terms of actuator design and trigger mechanisms for emergency operation.
The present invention relates to a so called fail safe actuator, i.e. an actuator which drives the valve it controls to the (process-wise) safer position out of two possible positions, on loss of power, or in response to a certain class of ESD. For instance, as applied to the Master or Wing Valve of a Xmas tree, the valve will go to the closed position on loss of control as part of a strategy to secure well safety barriers.
The task is thus to achieve an actuator which is very reliable in regular operation and which turns to a safe position on loss of active control. Historically the most widely accepted energy source to be activated on loss of control for safe closure of the critical XMT valves and other valves of similar functionality is in the release of a mechanical spring, which is kept pre-tensioned in the less safe valve position to provide the power source needed to move the slab gate against the forces of friction and other forces resisting the motion (closure or open as the case may be; some types of valves are to be opened during an emergency shutdown). For hydraulic actuators the fail safe mechanism is usually embedded in the electro-hydraulic control system in the form of a continuously energised solenoid valve. For the case of electrically operated actuators the spring is in most cases released by means of an electromagnetic device. In later years there are also examples of actuators reverting to the safe position in the ESD mode under a power supply from an electrical battery or other device for storage of electrical energy. Such designs usually depend on an electronic Variable Speed Drive (VSD) driving the motor for ESD operation, thus placing very high reliability requirements on these circuits.
For the purpose of the present invention it has been assumed that a mechanical spring is the most reliable energy source available.
One object of the present invention is to provide a highly reliable trigger mechanism for release of a spring action, thus maximising the reliability of the ESD process and thus securing a high SIL class.
There are many schemes devised to achieve a reliable trigger mechanism. This invention is based on a combination of an electrically controlled trigger with a leverage mechanism to trigger a spring action to close e.g. a Master Valve instantly if communication or power supply were to fail. The trigger mechanism is devised to handle the full force of the spring, thus avoiding involvement of any intermediate drive train components in the ESD process.
In many prior art actuators based on use of a mechanical spring to provide the emergency power the spring is moved with the gate motion at all times. This normally means that the valve is only actively driven to the least safe position (typically steady state production mode) and is always returned to the safe position under spring action. This approach only involves a motor and drive circuitry designed for single quadrant operation (active drive in one direction only, no generator operation). Other designs are based on pre-tensioning the mechanical spring only once and active drive of the valve back and forth. Obviously this is a two quadrant operation. The increase in complexity in the electrical/electronic circuitry to achieve the two quadrant operation is relatively insignificant. Also the inherent advantage of saving power by separating the motions of a roller screw and a spring is considered marginal and incidental. Reliability of the ESD function in combination with reliability of the regular operation represents the essence of any actuator design for the subject implementation.
The challenge in neutralising the spring force by resting it on a trigger mechanism lies in achieving a high reliability fail safe mechanism operating at low power consumption, for a spring of sufficient force. For some actuators required by the oil industry the required force may be as high as 40 tonnes, a typical order of magnitude for a 5″ slab gate valve, or even significantly higher for the case of 7″ valves.
It may be observed for instance in prior art U.S. Pat. No. 6,572,076 that the device for triggering the ESD requires the trigger power to act against the force of the main fail safe spring, thus quite substantial forces and power levels would be involved in countering the inherent force of the return spring in the steady state condition.
One object of the subject invention is to provide a gate valve actuator by which the power required for operation in the steady state condition being small and substantially less than the force exerted by the main return spring under compression, thus only counteracting a local and auxiliary spring force.
Another feature of referred prior art is the potential for galling effects as the gate operating stem is retracted upon shifting from steady state operation mode to shutdown mode.
It is another object of the subject invention to provide a gate valve actuator which is capable of releasing very high forces without galling effects.